1. Field of the Invention
The present invention relates to the field of liquid crystal displaying techniques, and in particular to a liquid crystal medium composition, a liquid crystal display panel manufactured with same.
2. The Related Arts
In the business of LCD (Liquid Crystal Display), the recently developed polymer stabilized vertical alignment (PSVA) technique shows various advantages, such as wide viewable angle, high contrast, and fast response, over the traditional twisted nematic (TN) and super twisted nematic (STN) techniques. As compared to other vertical alignment (VA) techniques, such as multi-domain vertical alignment (MVA) and patterned vertical alignment (PVA), PSVA shows advantages in respect of transmittance and simplified process. As a result, PSVA is the current main stream of the contemporary TFT-LCD business.
In a key process of manufacturing PSVA, as shown in FIG. 1, the PSVA technique uses a negative liquid crystal material. When no electric field is applied, liquid crystal molecules 106 are arranged in such a way of being substantially perpendicular to surfaces of upper and lower glass substrates 102, 100. An ITO slit 101 is formed in a TFT side of the lower glass substrate 100, while ITO 104 of the upper glass substrate 102 is intact. A certain amount of monomer, which when irradiated by ultraviolet light, carries out a polymerization reaction, is added to the liquid crystal material, and this is referred to as reactive mesogen (RM) 108. After an assembling operation is performed by having liquid crystal dropping onto the liquid crystal panel, a signal having proper frequency, waveform, or voltage is applied to the panel to cause the liquid crystal molecules to tilt in a predetermined direction. Meanwhile, ultraviolet light is applied to irradiate the panel to cause the RM 108 polymerized and forming a polymer deposited on the surfaces of the upper and lower glass substrates 102, 100. This polymer deposition formed through the polymerization is referred to as polymer bumps 110, which provide the liquid crystal molecules 106 with a pre-tilt angle with no voltage applied thereto. This helps increasing the response speed of the liquid crystal molecules. An advantage of using the technique is that multi-angle alignment (generating pre-tilt angles) in the panel can be realized.
Photo reactive groups contained in the reactive mesogens used in PSVA technique are usually methacrylate, acrylate, ethenyl, ethyleneoxy or epoxy groups, among which methacrylate is the most commonly used one. The primary wavelength range of ultraviolet light that is applied to cause photo-polymerization of the reactive mesogens of these functional groups is 200-300 nm. Although such reaction of the reactive mesogens may occur for ultraviolet light having a wavelength greater than 300 nm, yet the efficiency is extremely poor and the speed is low, making it poor in mass productivity. Consequently, it is necessary to use a light having a wavelength less than 300 nm to irradiate a panel for causing reaction of the reactive mesogens. However, a number of drawbacks and difficulties may be caused for the manufacture of panels if light of a wavelength less than 300 nm is used. Firstly, ultraviolet light having a wavelength less than 300 nm has higher energy, which may cause degradation and damage of polyimide that makes an alignment layer and the vertical alignment liquid crystal molecules used in this technique, leading to reduced VHR (Voltage Holding Ratio) of the panel, severe image sticking, and worsened result of reliability analysis (RA). Secondly, glass that is used to make upper and lower glass substrate of an LCD shows certain absorption of ultraviolet lights having wavelength less than 300 nm, causing deterioration of light irradiation efficiency. It is even worse that the liquid crystal material itself shows strong absorption of ultraviolet light having a wavelength less than 300 nm, as illustrated in the ultraviolet transmission spectrum of liquid crystal shown in FIG. 2. It can be found that ultraviolet light having a wavelength less than 300 nm is generally completely blocked from transmitting through the liquid crystal material. In other words, most of the ultraviolet light emitting from a light source is absorbed by the liquid crystal material (causing damage) and only a minor portion of the ultraviolet light is absorbed by the reactive mesogens to induce polymerization reaction. This occurs in a very shallow position at the light incidence side. This will cause inhomogeneous reaction of reactive mesogens between the light incidence side and the opposite back side, leading to worsened alignment effect of the panel.